In an optical communications system optical signals may be transmitted in free space, but are generally transmitted over optical waveguides, typically optical fibers. Since optical fiber transmission offers tremendous bandwidth and transmission rate advances over the transmission of electrical signals, conversion to electrical signals are avoided as much as possible by active optical processing such as optical amplification, switching and routing. It is usually desirable to avoid conversion of the signal to an electrical signals until they reach the target destination, where they are converted back to electrical signals representing digital data, voice or images in various analog formats.
In order to maximize the capacity of fiber optic communication systems many signal are simultaneously transmitted over the same fiber waveguides in a scheme known as wavelength division multiplexing or WIDM. In WDM each discrete signal may correspond to a different wavelength of light, known as an optical channel. Various non-linear properties of optical glass, active and passive components in the optical system, produce cross talk between the WDM optical signal channel. This “cross talk” is insignificant if the signal to noise ratio is high and the power levels of all optical channels are comparable.
The optical devices and interconnections in any route will result in signal losses, thus the signal power and signal to noise ratio of any optical signal can be expected to vary with the routing path. When the communication system is a network, optical channels are combined and routed together in common waveguides with signals from different sources the power levels in each optical channels are likely to be different, in which case the “cross-talk” from the stronger channels will degrade the signal to noise ratio in the weaker channels.
Therefore, low insertion and high isolation is a substantial consideration in the design and operation of all optical communication system components. While very low losses can be obtained by fusion splicing optical fibers of similar composition many passive and active components preclude direct connections because of intermediate components, such as filters, mirrors or prisms, which route or multiplex/de-multiplex the optical signal channels. In a typical device a single mode optical fiber is connected to the device at a first, or input, port and one or more additional single mode optical fibers are connected at additional ports. Light exiting the optical fiber at an input port is collimated into a substantially parallel beam by a lens. Additional lenses located at the output ports converge the collimated beam into the outgoing optical fiber connected thereto. Lateral and angular offsets of the collimating elements contribute to the signal loss. Since the collimated beam diameter is many times the diameter of the fiber core, typically 10 microns, the signal loss due to lateral offset is reduced. However, the sensitivity of signal loss to angular offset increases with beam diameter.
However, the typical macroscopic collimated lenses present limitations in miniaturizing devices or increasing the interconnection density without increasing the device or package size considerably. While several methods have been suggested for fabricating a lens on the end of a single mode optical fiber they are not suitable when there must be very low signal loss or a miniature device, such as optical cross-connect switches or multiplex/de-multiplex device.
Several patents describe how a refractive surface of micro-lenses can be formed or attached to the surface of a single mode fiber. In U.S. Pat. No. 4,268,112 to Paterson a Luneberg type lens with a gradient of refractive index is attached to the end of an optical fiber, however the lens diameter is larger than the fiber diameter. In U.S. Pat. No. 4,205,901 to Ramsey et al. a single mode fiber is terminated with a core end region having a core with a graded composition and increasing thickness towards the end of the fiber. In U.S. Pat. No. 4,456,330 to Bludau a homogeneous glass rod is welded to the end of a fiber and rounded by heat treatment to form a hemispherical lens. However, these design either have significant disadvantages with respect to achieving a high interconnection density devices, for example the formation of an adequate lens either increases the diameter of the single mode fiber, or distorts the edge, thus making the subsequent alignment necessary to achieve low insertion loss difficult, or have a high return loss. The additional components increase the complexity of assembly and result in additional signal loss from splice misalignment.
Another approach to forming a single mode fiber with a micro-lenses function is to fuse a short section of multimode optical fiber to the terminal end of a single mode fiber wherein the multimode fiber acts as a gradient index lens, such as in U.S. Pat. No. 4,701,011 by Emkey et al. Alternatively the refractive index may be tapered linearly, such as in U.S. Pat. No. 4,737,004 to Amitay et al., or U.S. Pat. No. 5,337,380 to Darbon et al. However, it has been found that such devices are not suitable in miniature devices because they cannot easily be aligned, due to irregularities in the surface shape at the fusion joint, and/or do not shape the exiting beam in a manner compatible with both low loss and a high-density of interconnection.
In U.S. Pat. No. 6,014,483 Thual et al. teach that it is possible to increase the working of distance of—coupler taught by Emkey et al. by adding a silica spacer between the single mode fiber and the multimode. U.S. Pat. No. 5,457,759 to Kalonji et al. discloses combining in succession: a piece of graded index multimode fiber, a piece of step index multimode fiber and a micro-lens, wherein the terminating micro-lens is a curved refracting surface. However, such configurations appear too difficult to manufacture without increasing or distorting the outer diameter, which is problematic in alignment and assembly. Furthermore, such combinations suffer undesirable back reflection or return loss.
Accordingly, it is an object of the current invention to provide a compact optical fiber coupler suitable for the miniaturization of high-density interconnection devices.